A number of mechanisms for locking door locks having conventional square-section spindles have been proposed in the past. U.S. Pat. No. 991,018 (Powers) and GB-364,396 (Freeman) disclose the use of a U-shaped locking plate which in use is slid over the spindle to stop its rotation and the release of the door. However, on both of these, the locking plate is only releasable from one side of the door. Other door lock mechanisms are disclosed in U.S. Pat. No. 2,125,518 (Oldham), U.S. Pat. No. 2,295,435 (Teich), U.S. Pat. No. 2,400,403 and GB No. 322,195 (Good).